ZrO2 Crystals Research Articles

Diffusion bonding of niobium and Y2O3-stabilized ZrO2 or HfO2.

Y2O3-stabilized ZrO2 or HfO2 single crystals with (100), (110), or (111) surface are diffusion-bonded to polycrystalline niobium, and then joints are tensile tested. The bond strength is dependent on the surface orientation of ZrO2 or HfO2: higher bond strength is obtained in the (100) and (110) surfaces than in the (111) surface. Although any reaction layers are not observed by EPMA at the bonding interface, black dots are found on the torn surfaces of both the ceramic and the metal in the joints bonded at 1 773 K and above and then tensile tested, which may imply that some reaction has occurred at the bonding interface. Lattice matching is also considered for observed crystallographic orientation relationships.

Solution-Derived Yba2Cu,Sub3O7−Δ. Thin Films And Barrier Layers

ABSTRACTHydrolyzed metal alkoxide solutions were spin-coated on Si substrates with subsequent thermal processing. Barrier layers of alkaline earth oxides, perovskites, Y2O3, ZrO2 and others were produced. Characterization was performed via SEM, XRD, ESCA, Auger depth profiling and resistivity measurements. Barrier layer films were fairly smooth with some cracking and pitting present. Si migration was severe for alkaline earth thin films on Si wafers. Some perovskite films on Si showed formation of Ba-Si-O phases at the Si interface. Thin films of 1–2–3 on barrier layers of SrTiO3 or ZrO2 on Si showed Ba pileup at the Si interface and were not superconducting. 1–2–3 layers deposited on single crystal ZrO2 were superconducting and showed onset temperatures of 90 K with zero resistance reached at about 55 K.

Electrical characteristics of metal-insulator-semiconductor diodes with ZrO2/SiO2 dielectric films

Thin films of ZrO2 vacuum-deposited on Si(100) substrates were oxidized in dry O2 at 800 °C and then analyzed by infrared absorption and Auger electron spectroscopy. It was found that the SiO2 layer was formed at the ZrO2/Si interface. Mo/ZrO2/SiO2/p-Si(100) metal-insulator-semiconductor diodes were electrically characterized. The static dielectric constant of the ZrO2 layer decreases from 21.1 ε0 to about 15.5 ε0 due to the crystallization of amorphous ZrO2 during the oxidation. The SiO2 layer formed at the ZrO2/Si interface was found to lower the leakage current through the dielectric film.

A computer graphic investigation on the epitaxial growth of Ba2YCu3O7-x superconductor films.

An epitaxial growth mechanism of oxide superconductor films has been investigated by using computer graphics for the combination of orthorhombic Ba2YCu3O7-x with substrate crystals of SrTiO3, MgO, and ZrO2. The (001) plane of Ba2YCu3O7-x has been shown to fit well the (100) plane of these substrate crystals. A crystallographic fit has also been demonstrated between the (110) plane of Ba2YCu3O7-x and the (110) plane of SrTiO3. Detailed investigation of atomic arrangements has indicated some differences in the ionic interaction at the superconductor-substrate interface among SrTiO3, MgO, and ZrO2 substrates. For the combination of the (001) plane of Ba2YCu3O7-x and the (100) plane of SrTiO3, various layers of Ba2YCu3O7-x and SrTiO3 are possible to provide arrangements of ions which stabilize the electrostatic interaction at the interface. Although the Ba-O layer of Ba2YCu3O7-x does not provide such a stable interaction with the MgO (100) plane, the Cu-O layer of Ba2YCu3O7-x can favorably interact with the MgO (100) plane. As for ZrO2 (100) plane, ionic arrangements at the oxide layer is favorable only for the interaction with Y3+ layer of Ba2YCu3O7-x while the Zr-O layer of ZrO2 can interact with both Ba-O layer and Cu-O layer of Ba2YCu3O7-x.

ChemInform Abstract: Polymorphic Mixes of Zirconia from a Borate Glass Melt.

AbstractYttria‐stabilized ZrO2 crystals are prepared by devitrification of a sodium borate glass containing zirconia and yttria in varying ratios (85 wt.% B2O3, 5 wt.% Na2O, 10 wt.% (ZrO2 + Y2O3)).

Microstructural Characterization and Fracture Toughness of Cordierite‐ZrO2 Glass‐Ceramics

Crystallization of an MgO‐Al2O3‐SiO2‐ZrO2 sintered glass frit was studied. Heat treatment at 850° or 900°C caused initial crystallization of μ‐cordierite and tetragonal (t) ZrO2. The t‐ZrO2 crystallized with an irregular dendritic morphology and could be transformed to monoclinic (m) symmetry under certain conditions; the cordierite underwent the μ→α a transformation with extended annealing. Heat treatments at 1000°C caused crystallization of t‐ZrO2 rods and spheroids in an α‐cordierite matrix; these ZrO2 crystals, however, are resistant to transformation to m‐ZrO2. The beneficial effects of ZrO2 on the fracture toughness of cordierite‐based glass‐ceramics are described.

Formation Reaction of ZrO2 Scatterers in ZrF4-Based Fluoride Fibers

This paper clarifies the formation reaction of ZrO2 crystals which appear as extrinsic scatterers in fluoride fibers. EPMA analysis indicates that BaO exists at grain boundaries of BaF2 purified by sublimation. BaO reacts with ZrF4 to form ZrO2 at 600°C during a glass-melting process. The ZrO2 formation reaction is influenced by H2O. Ba(OH)2, which is formed by the reaction between BaO and water vapor, melts at 370° to 420°C and reacts with ZrF4 to form ZrO2 at 450° to 520°C. When low-oxide-content BaF2 is used for fiber preparation, scatterers significantly decrease.

Thermal Diffusivity of Partially and Fully Stabilized (Yttria) Zirconia Single Crystals

Laser flash measurements of thermal diffusivity of ZrO2 single crystals partially and fully stabilized with Y2O3 were compared with measurements for polycrystalline cubic ZrO2, and single crystals and polycrystals of Al2O3 and MgAl2O4. In general, the thermal diffusivities of the ZrO2 materials examined initially decrease with increasing temperature, although significantly less than for the Al2O3 and MgAl2O4 materials. The diffusivity subsequently rises with increasing temperature for the single crystals of ZrO2 but not for polycrystalline cubic ZrO2, with this increase for ZrO2 crystals occurring at much lower temperatures than for Al2O3 and MgAl2O4 crystals. As the ZrO2 materials went from fully stabilized with 20 wt% Y2O3 to partially stabilized with 5 wt% Y2O3, the room‐temperature diffusivity increased from 0.70x10‐6 to 0.97x10‐6 m2/s. This indicated that the lattice defects, which increase with Y2O3 content, are more important sources of phonon scattering than are the precipitates which appear at lower Y2O3 compositions. On the other hand, at 1000°C the diffusivity of the fully cubic ZrO2 crystals with 20 wt% Y2O3 was 1.10x10‐6 m2/s while that of the partially stabilized ZrO2 crystals with 5 wt% Y2O3 was 1.05x10‐6 m2/s. The increased diffusivity at higher temperatures can be attributed to radiative heat transfer, with the lower relative diffusivity of the partially stabilized crystals resulting from photon scattering by the precipitate structures present.

Synthesis of Separate Polycrystalline PbTiO<sub>3</sub>-PbZrO<sub>3</sub> Films by Sol-Gel Process and Their Orientations

ZrO2, TiO2 and ZrTiO4 films were prepared from sols of crystalline fine particles of ZrO2 and TiO2 by drying on glass substrates and firing gel films which were separated from the substrates. Then separate polycrystalline films (10-20μm thick) of PbZrO3, PbTiO3 and Pb (Zr, Ti) O3 were synthesized by heating ZrO2, TiO2 and ZrTiO, films, respectively, with lead acetate or lead oxide. Grain orientation was observed in the PbZrO3 and PbTiO3 films, but not in the Pb (Zr, Ti) O3 film. The orthorhombic b-axis in the PbZrO3 film and the tetragonal c-axis in the PbTiO3 film were perpendicular to the film plane. The orientation of PbZrO3 depended on the orientation of ZrO2 crystals, whereas the orientation of PbTiO3 was independent of orientation of TiO2 crystals and depended on the flatness of the polycrystalline surface of TiO2. It was supposed that the oriented PbTiO3 crystals are formed by crystallization and growth in a thin liquid phase on the film surface.

Hydrolysis-Condensation of Zr-O-Si Alkoxides

A Zr-O-Si alkoxide complex was synthesized and its hydrolysis-condensation reaction was studied using small angle X-ray scattering. Transparent and homogeneous Zr-O-Si alkoxide solutions were formed by reacting Zr(OC3H7)4 with Si(OC2H5)4 hydrolyzed with one mole of water per one mole of Si(OC2H5)4 and HCl catalyst, whereas the solutions hydrolyzed with NH4OH catalyst or without any catalyst resulted in precipitation of ZrO2 crystals. Homogeneous Zr-O-Si alkoxide solutions were hydrolyzed with 0 to 8 moles of water to metal alkoxide, whose X-ray scattering curves were analyzed in both power law and Guinier law. The synthesized Zr-O-Si alkoxide consisted of chain like polymers, which were aggregated into three dimensional branched polymers with water content up to 4 moles per 1 mole metal alkoxide and their size increased up to 65A in the Guinier radius. However, no further changes in size and shape of aggregated polymers were observed with more than 4 moles of water.

Scattering characteristics of crystallites in fluoride optical fibers

This paper evaluates the scattering loss in ZrF(4)-based fluoride optical fibers that is caused by ZrF(4) and ZrO(2) crystallites. To determine this, efficiency factors of each of these crystals are calculated. We make clear that the presence of ZrO(2) crystals of the order of a submicron smaller than 0.2 microm in diameter effectively elevates the Rayleigh scattering factor as the particle number increases. Scattering loss characteristics in current fluoride fibers are also discussed. In addition, ZrO(2) microcrystals in fluoride fibers are identified for the first time using Raman microprobe analysis. It is concluded that ZrO(2) microcrystals cause serious extrinsic scattering loss in current ZrF(4)-based fluoride fibers.

ZrO<sub>2</sub> Transformation of Glass-Ceramics in the System ZrO<sub>2</sub>-SiO<sub>2</sub> Prepared by the Sol-Gel Process from Metal Alkoxides

Crystal growth of ZrO2 and crystallite size dependence of tetragonal to monoclinic ZrO2 transformation were investigated in relation to the fracture toughness of glass-ceramics. Glasses of 2ZrO2⋅3SiO2, ZrO2⋅SiO2 and 3ZrO2⋅2SiO2 in molar ratio, prepared by the sol-gel process from metal alkoxides, were heat-treated to precipitate tetragonal (t-) ZrO2 crystals. Tetragonal-ZrO2 crystals grew in proportion to the cube-root of heat-treatment time, and the growth rate increased with increasing ZrO2 content. Crystals of t-ZrO2 larger than a critical size transformed into monoclinic (m-) ZrO2 during cooling. Transmission electron microscopy revealed that the m-ZrO2 particles showed twinning associated with t- to m-ZrO2 transformation. Tetragonal to m-ZrO2 transformation temperature was found proportional to the reciprocal ZrO2 crystal size, which was consistent with Garvie's transformation equation. The critical sizes corresponding to the transformation temperature of 273K were estimated to be 90, 56 and 40nm for 2ZrO2⋅3SiO2, ZrO2⋅SiO2 and 3ZrO2⋅2SiO2 glass-ceramics, respectively. The interfacial and strain energies for the transformation calculated from Garvie's equation increased with decreasing ZrO2 content. The fracture toughness (KIC) of these glass-ceramics increased in proportion to the cube of t-ZrO2 crystal size. From the critical crystal size of t-ZrO2 in the glass-ceramics, the maximum KIC was estimated to be 4.7, 4.7 and 5.0MN/m3/2 for 2 ZrO2⋅3SiO2, ZrO2⋅SiO2 and 3ZrO2⋅2SiO2, respectively. After reaching the maximum, KIC decreased abruptly on further heating, which was attributed to the occurrence of m-ZrO2 crystals in the glass-ceramics.

Size Control of ZrO<sub>2</sub> Aggregated Particles by Chloride-Coexisted Hydrolysis and Sintering Them

The effect of metal chlorides added to ZrOCl2 solutions on the crystallization of monoclinic ZrO2 in high temperature hydrolysis was studied. It was found that an increase in the amount of chloride ions decreases the crystallinity and forming rate of ZrO2 and increases the size of ZrO2 aggregated particles. The ZrO2 aggregated particles were grown to about 0.4μm by hydrolysis at 150°C in a solution containing 0.2mol/l of ZrOCl2 and 2mol/l of AlCl3. In the preparation of 3mol% Y2O3-TZP, compacts of ultrafine powder obtained by hydrolysis densified at lower temperatures, along with larger shrinkage due to lower compactability. On the other hand, aggregated particles of about 0.4μm were well isolated and did not form further strong aggregates each other at calcination up to 1000°C, and then resulted in higher green density. The compacts did not densify at lower temperatures but sintered to nearly theoretical density with about 0.4μm grains at 1400°C.

Crystallization of Mullite-Zirconia Amorphous Materials Prepared by Rapid-Quenching

Amorphous materials have been prepared by rapid quenching, -105°C/s, of the melt of the mixed oxides, 20wt% ZrO2-80wt% (3.4Al2O3⋅2SiO2) using an arc-imaging furnace and a twin-roller. This paper deals with the crystallization and the products of these amorphous materials obtained by repeating under various conditions. The crystallization of this amorphous material took place at -990°C precipitating t-ZrO2 and mullite phases after showing glass-transition temperature at 919°C on heating by 10°C/min. The mullite had markedly larger lattice parameters than that of stoichiometric mullite (3Al2O3⋅2SiO2) indicating higher (>83wt%) Al2O3 contents in the early stage of crystallization, then showed smaller lattice parameters on prolonged reheating. The TEM observation of the sample re-heated at 1200°C for 12h revealed that the product consists of mullite grains of 2-5μm in which fine (10-20nm) t-ZrO2 crystals are contained. The characteristic arrangements of the fine ZrO2 crystals in the mullite grains suggest that these t-ZrO2 crystals are precipitated by exsolution on the crystallization of the mullite from the amorphous matrix. Reeating of the amorphous materials at 880°C reduced the glass-transition and crystallization temperatures and increased the lattice parameters of mullite. This suggests that the heat treatment at 880°C caused a phase separation in amorphous samples. The phase separation would yield the precipitation of Al2O3-rich mullite in the early stage of crystallization with the exsolution of t-ZrO2 micro-crystals.

Crystallization of mullite-zirconia amorphous materials prepared by rapid-quenching

Amorphous materials have been prepared by rapid quenching, -105°C/s, of the melt of the mixed oxides, 20wt% ZrO2-80wt% (3.4Al2O3⋅2SiO2) using an arc-imaging furnace and a twin-roller. This paper deals with the crystallization and the products of these amorphous materials obtained by repeating under various conditions. The crystallization of this amorphous material took place at -990°C precipitating t-ZrO2 and mullite phases after showing glass-transition temperature at 919°C on heating by 10°C/min. The mullite had markedly larger lattice parameters than that of stoichiometric mullite (3Al2O3⋅2SiO2) indicating higher (>83wt%) Al2O3 contents in the early stage of crystallization, then showed smaller lattice parameters on prolonged reheating. The TEM observation of the sample re-heated at 1200°C for 12h revealed that the product consists of mullite grains of 2-5μm in which fine (10-20nm) t-ZrO2 crystals are contained. The characteristic arrangements of the fine ZrO2 crystals in the mullite grains suggest that these t-ZrO2 crystals are precipitated by exsolution on the crystallization of the mullite from the amorphous matrix. Reeating of the amorphous materials at 880°C reduced the glass-transition and crystallization temperatures and increased the lattice parameters of mullite. This suggests that the heat treatment at 880°C caused a phase separation in amorphous samples. The phase separation would yield the precipitation of Al2O3-rich mullite in the early stage of crystallization with the exsolution of t-ZrO2 micro-crystals.

Crystal growth of tetragonal ZrO2 in the glass system ZrO2-SiO2 prepared by the sol-gel process from metal alkoxides

Glasses in the system ZrO2-SiO2 containing 40 to 60 mol % ZrO2 were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO2 was precipitated by heat treatment at 800 and 1200° C, and its crystal growth was measured by differential thermal and X-ray diffraction analyses. At 800 to 900° C, tetragonal ZrO2 crystals grew three-dimensionally and the activation energy for growth was calculated as about 680 kJ mol−1. On the other hand, the secondary growth of tetragonal ZrO2 at 1000 to 1200° C followed the cube-root-of-time law. The activation energy for secondary growth was about 380 kJ mol−1. It is suggested that the diffusion of Zr4+ ions is the rate-limiting process for the secondary crystal growth of tetragonal ZrO2.

ChemInform Abstract: ZrO2‐Transformation‐Toughened Glass‐Ceramics Prepared by the Sol‐Gel Process from Metal Alkoxides.

AbstractGlass of 3 ZrO2 ‐ 2 SiO2 composition, prepared by the sol‐ ‐gel process, was crystallized to precipitate tetragonal ZrO2 crystals.

The Temperature Dependence of Yield Stress and Fracture Toughness in Unstabilized Zirconia Crystals

ABSTRACTThe flexural strength and the single edge notch beam fracture toughness of undoped ZrO2 crystals, grown by the skull melting technique, were examined from room temperature to 1400°C. On heating the toughness increased with test temperature to a maximum of 4.0 MPajm at 1225°C then gradually decreased to 2.6 MPa/m. Upon cooling after a 20 minute hold at 1250°C an increase in toughness to 5 MPa/m was observed at 1200°C; upon cooling to lower temperatures Kic gradually diminished. The loaddeflection curves for the flexural strength tests showed marked nonlinearity before failure for samples tested on cooling. The temperature dependence of the apparent yield stress suggests that initial yielding occurs by slip above 1200°C but that from 1200°C to 1050°C the observed yielding is due to stress induced tetragonal to monoclinic transformation.

The reassociation of plasma dissociated zircon

Plasma dissociated zircon, which consists of finely divided spherulitic crystals of monoclinic ZrO2 in SiO2 glass, reassociates very slowly at 1500° C unless finely ground. It is suggested that this effect is due to surface nucleation control of the reaction rate. Plasma dissociated zircon in the form of a rapidly quenched coating consists entirely of tetragonal ZrO2 crystals less than 20 nm diameter dispersed in glass. Reassociation occurs rapidly between 1300 and 1450° C because of the surface nucleation effect and enhanced zircon crystal growth rate arising from the very small diffusion distance ahead of the interface of a growing zircon crystal.

Crystallization process of cubic ZrO2 in calcium acetate solution

Ultrafine powders of cubic ZrO2 were obtained at about 270° C by heating hydrated amorphous ZrO2 in greater than 0.2 molal calcium acetate solutions. Ca2+ ions played a role as nucleii for crystallization and were introduced into distinct sites of the crystalline phases, that is, substituted for Zr4+ ions. Mn2+ ions produced almost the same effects on the crystallization of ZrO2·EPR spectra for powder samples containing Mn2+ ions apparently showed two types as follows: for tetragonal ZrO2 with a trace of monoclinic ZrO2, the central fine structure transitions (M=+1/2↔−1/2) showed a well-resolved hyperfine structure. In addition to the Δm=0 transition, forbidden Δm=±1 transitions were observed. For cubic ZrO2, the broad underlying response was observed as well as the hyperfine structure composed of six main peaks.